If your preboil gravity was right, and that reading came from the proper preboil amount of wort then something is wrong. Either you way overestimated your evaporation rate and made more beer than expected or you got a bad reading. Did you adjust for temperature when you took both gravity readings?

If your preboil gravity was right, and that reading came from the proper preboil amount of wort then something is wrong. Either you way overestimated your evaporation rate and made more beer than expected or you got a bad reading. Did you adjust for temperature when you took both gravity readings?

I may have overestimated my evaporation rate, as it is usually extremely high, but I tried to keep the burners low enough to just maintain a rolling boil.

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"I don't understand why everybody is focusing only on details. It is impossible to produce a good beer with details."
Jean-Marie Rock, head brewer at Orval

One more testament to the value of a sight tube on your kettle. We can take gravity readings all day long but without accurate volume measurements, they don't mean much. Monitoring your volume during the boil really helps to moderate your boil-off rate. My typical boil-off rate is 1.5 gallons per hour which is 1/2 gallon every 20 minutes. My sight tube is calibrated in 1/2 gallon increments so I can glance at it every 10 minutes or so to gage how to adjust the heat.

I have noticed too, that when I substitute a liquid form of "sugar" (honey, LME) for a dry form (DME, corn sugar, etc) that I usually don't quite hit my OG, since I forget to take into account the amount of water that is in the "sugar" that I just added. I guess it is all about good record keeping and good measurements... that, or just years of experience and being able to "eyeball" it...